The present invention relates to the general field of material for forming abradable coverings. A particular application lies in the field of turbine engines where use is made of abradable seals.
Materials having an abradable property are commonly used in numerous applications, and in particular for forming seals. Abradable seals are used in particular in the rotary portions of a turbine engine, such as in the compressors, in order to reduce leaks of air or gas that might affect the efficiency of the engine. A turbine engine compressor comprises a plurality of blades fastened on a shaft that is mounted inside a stationary ring. In operation, the shaft and the blades rotate inside the compressor ring.
In order to guarantee good efficiency for the turbine engine, it is important to reduce leaks of air or gas in the compression sections of the engine to as little as possible by minimizing the clearance that exists firstly between the tips of the blades and the inside surface of the compressor ring, and secondly between the inter-disk shrouds and the outside surface of the diffuser. Because of thermal and centrifugal expansion of compressor blades, the inside surface of the compressor ring is generally covered in a covering of abradable material so as to form a seal between the stationary portions (specifically the inside surface of the ring) and the moving portions (specifically the tips of the blades) of the compressor of a turbine engine. In the event of contact between the stationary and moving portions of the compressor, the seal made of abradable material makes it possible to obtain clearance that is small without that damaging the parts of the rotor that make contact.
The abradable coverings used in turbine engine compressors are generally made from a composition comprising an alloy of aluminum and silicon (Al—Si) and a polymer such as a polyester, the composition being deposited directly on the wear portion of the part by thermal spraying. In that manner, an abradable covering is obtained that is formed by a matrix of Al—Si alloy, i.e. a continuous phase of the alloy in the material, together with a filler of polyester.
Nevertheless, that type of covering is found to be very sensitive to corrosion, in particular when it is exposed to saline environments or to atmospheric pollution. Corrosion is also made considerably worse when the part, as a result of its shape, e.g. annular, includes retention zones that amplify exposure of the abradable covering to corrosion agents. These corrosion phenomena have the effect of reducing the lifetime of the abradable covering.
Solutions that have been used in the past for reducing the corrosion phenomenon on that type of abradable covering are as follows:                protecting the abradable covering with a varnish or with an anticorrosion paint;        waterproofing or draining the retention zones; and        modifying the abradable composition.        
Nevertheless, each of those solutions presents drawbacks. Anticorrosion paints or varnishes serve to protect the covering only on its surface and they cease to be effective once they are worn away. They also increase the cost and the time required for fabricating abradable coverings. Draining retention zones requires holes to be made in the part, thereby weakening its mechanical strength and encouraging leaks of air. Waterproofing increases the cost of fabricating the abradable covering (additional material) and makes mounting and removing parts more complex. Finally, the use of a different abradable covering composition, as described in particular in Document U.S. Pat. No. 7,160,352, implies, in aviation, that the covering needs to be qualified, and that constitutes a major constraint.